A Tunnel Sloping Downwards                             -      N      - 
(-------------------------------------------------)     - <---(M)---> E
                                                        -      -      -

You are walking along a tunnel, that slopes downward to the east.  The
walls are cut from the dirt, and as you touch one side, a small piece of
dirt falls down.  The ceiling is relatively low, and the ground very hard. 
The tunnel is very narrow here, and the ceiling stands high above your head.
To the east the tunnel gets darker and darker.